Marine Quadrajet Issue - Will not idle when hot

[rudie20]

Hi all.  Looking for some help for my new (used) boat.  I bought at the end of season last year and am struggling keeping the engine running when hot.  The quadrajet part number is 17085013.

Items I know:
 - When cold, the boat fires right up and runs great
 - Once the engine is hot, it dies at idle when trying to put boat into gear
 - I can start in neutral if I crank the fast idle to nearly full throttle.  It will then fast idle great at any rpm above idle.
 - Ether will make the engine die faster at idle (running rich)
 - Seems to run best while warming up with idle screws turned all the way in, blocking all fuel bypass
 - Rebuilt the carb last year with a rebuild kit.  Replaced gaskets, float (did not need adjustment), and cleaned all jets.  The AP was a different size in the rebuild kit than the original so I did not replace (no rips or tears in gasket).  After some research, I think that the smaller AP will actually work so I plan to replace.
 - It is clear by the paint that the carb had never been rebuilt previously
 - After rebuild, same symptoms persist.

After some research, I am thinking that maybe the issue is one of two things:
1. Maybe the AP is leaking fuel around the gasket and causing the carb to run rich when it heats up and expands?
2. Bottom plugs leaking when hot?

Can anyone think of any other reasons for these symptoms?

Thanks

[77cruiser]

Have you checked the fuel pressure?

[rudie20]

I have not.  I will as soon as I get the boat back in the water this year.  Would the fuel pressure change as the engine gets hot? 

[Cliff Ruggles]

If the accl pump was too long you did not get a dedicated Marine rebuild kit.  I'll bet the N/S assembly was also incorrect, the Marine carb requires the high flow assembly.

Could be several things going on.  I would get one of our Marine kits and go back thru the carb.  This will insure the correct and upgraded internals are in place.  I don't care what they tell you or who's selling them, the Viton pump cups and N/S assemblies in the kits are very low quality and we see a lot of failures with those items.

While the carb is apart, pressure test the bottom plugs and install screw in plugs with our bottom plug kit if/as needed.

Using the old spring under the pump, "rod" out the idle tubes or pull them and manually clean them with a precision drill bit in a pin vise.

"Blubbering" at idle and having to seat the mixture screws indicates too much fuel available at idle speed, most likely "nozzle dripping" from insufficient fuel from the idle system, or fuel level to high from a leaking N/S assembly or the gasket under it.  Float may be "heavy" as well. 

We get a LOT of freshly "rebuilt" carbs in here with similar issues, and more times than not it's just crappy parts combined with plugged up idle tubes or leaking bottom plugs.......Cliff

[rudie20]

Hi Cliff. Got back my qjet that I sent in to you for rebuild but am still having same issue . One thing to note is that the gasket that you sent for between the carb and manifold is way thicker (about 1/4") than the original (about 1/16") so I had to purchase another stock gasket or the bolts were too short.

A lot of people have suggested the fuel pump as being the problem but this doesn't make sense to me. The carb is running rich at idle and well at WOT so wouldn't that mean there is ample if not too much fuel? Do fuel pumps fail with too much pressure?

Thanks

[Cliff Ruggles]

We've seen a lot of fuel pumps that are putting out too much pressure and causing issues.

We tested the carb on our test engine with 6psi fuel pressure and no issues anyplace.....Cliff

[rudie20]

Thanks all.  You were right! Fuel pump was the issue. Overpressurizing fuel system at idle.

[77cruiser]

What was the pressure?